Convicted child killer Robert Black has died in prison in Northern Ireland aged 68, prison sources have said.

Northern Ireland Prison Service confirmed an inmate died in non-suspicious circumstances.

Prison sources have said the dead prisoner was Black.

The Scottish-born serial killer was serving 12 life sentences for the kidnapping and murder of four girls between the ages of four and 11 between 1981 and 1986 in the United Kingdom.

He was convicted of sexually assaulting one of the girls, and raping the other three. The killer was also convicted of a kidnapping of a fifth young girl and the attempted kidnapping of another.

He was named as a suspect in the unsolved disappearance of Donegal girl Mary Boyle who vanished 39 years ago, aged six.

Mary was reported missing by her distraught parents on Friday, March 18, 1977. Mary had been at her grandparents' house in Cashelard, a remote and boggy townland outside Ballyshannon, where the extended family had lunch when she went to walk up the fields. She never returned.

Black is also suspected of several other unsolved child disappearances or murders in the UK and Europe in the 1970s.

Convicted child killer Robert Black was a suspect in her disappearance because he travelled from Britain to Ireland as a van driver and spent time in the Donegal area.